1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a fixture and method for inserting a locking tube into a locking position in a removable top nozzle of a reconstitutable fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle. The guide thimbles may each include an upper sleeve for attachment to the top nozzle.
During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus estabishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of the integral assembly of guide thimbles welded to the top and bottom nozzles, it is difficult to detect and remove the failed rods.
Until recently, to gain access to these rods it was necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in the reactor because of the damage done to both the guide thimbles and the nozzle which prohibits rewelding.
In view of the high costs associated with replacing fuel assemblies, considerable interest has arisen in reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. One reconstitutable fuel assembly construction, devised recently, is illustrated and described in the first U.S. patent application cross-referenced above. It incorporates an attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles.
The attaching structure includes a plurality of outer sockets defined in an adapter plate of the top nozzle, a plurality of inner sockets with each formed on the upper end of one of the guide thimbles, and a plurality of removable locking tubes inserted in the inner sockets to maintain them in locking engagement with the outer sockets. Each outer socket is in the form of a passageway through the adapter plate which has an annular groove. Each inner socket is in the form of a hollow upper end portion of the guide thimble having an annular bulge which seats in the annular groove when the guide thimble end portion is inserted in the adapter plate passageway. A plurality of elongated axial slots are provided in the guide thimble upper end portion to permit inward elastic collapse of the slotted portion so as to allow the larger bulge diameter to be inserted within and removed from the annular circumferential groove in the passageway of the adapter plate. In such manner, the inner socket of the guide thimble is inserted into and withdrawn from locking engagement with the outer socket.
The locking tube is inserted from above the top nozzle into a locking position in the hollow upper end portion of the guide thimble forming the inner socket. When inserted in its locking position, the locking tube retains the bulge of the inner socket in its expanded locking engagement with the annular groove and prevents the inner socket from being moved to a compressed releasing position in which it could be withdrawn from the outer socket. In such manner, the locking tubes maintain the inner sockets in locking engagement with the outer sockets, and thereby the attachment of the top nozzle on the upper ends of the guide thimbles. Furthermore, due to vibration forces and the like, it is desirable to secure the locking tubes in their locking positions. For such purpose, suitable means, such as a pair of bulges, are formed in the upper portion of each locking tube after insertion in its locking position which bulges fit into the circumferential bulge in the upper end portion of the guide thimble.
Prior to removal of the top nozzle from, and after its replacement back on, the fuel assembly, the locking tubes must be removed from and replaced back at their locking positions. One tool for accomplishing either removal or replacement of each locking tube, individually or, in other words, one at a time, is illustrated and described in the second U.S. patent application cross-referenced above. However, it is believed that some customers may perceive the individual handling of locking tubes during reconstitution as posing a risk of tubes escaping and somehow becoming lost in the fuel assembly. This possibility is extremely remote since locking tubes are only handled and transported over a fuel assembly when the nozzle is mounted on the assembly, and no opening through the nozzle or in the annulus between the assembly and the work station is sufficiently large to allow passage of a dropped locking tube.
Notwithstanding the acceptable performance of the above-mentioned tool in removing and replacing locking tubes and the unlikelihood of losing a locking tube in the fuel assembly when using such tool, the possibility of customer concern over individual locking tube handling coupled with the desire to accomplish removal and replacement of the top nozzle in a shorter period of time have created a need for further improvement of the reconstitution operation so as to enhance commercial acceptance thereof.